Many methods are known for chemical sensitization of silver halide photographic emulsions (hereinafter referred to more simply as "silver halide emulsions").
For instance, (1) a sulfur sensitization process is known wherein a compound containing sulfur capable of reacting with silver ions or active gelatin is used, (2) a reduction sensitization process is known wherein a reducing substance is used, and (3) a noble metal sensitization process is known wherein a gold compound or other noble metal compounds are used. These processes can be used individually or in combination with each other.
Sulfur sensitizers that can be used include thiosulfates, thioureas, thiazoles, rhodanines, and other various compounds. Practical examples of such sensitizers are described in U.S. Pat. Nos. 1,574,944; 2,410,689; 2,278,947; 2,728,668; 3,656,955; 4,030,928; and 4,067,740.
As reduction sensitizers, stannous salts, amines, hydrazine derivatives, formamizinesulfinic acids, silane compounds, etc., can be used, and practical examples thereof are described in U.S. Pat. Nos. 2,487,850; 2,419,974; 2,518,698; 2,983,609; 2,983,610; 2,694,637; 3,930,867; and 4,054,458.
For noble metal sensitization, gold complex salts, as well as other complex salts of metals belonging to group VIII of the Periodic Table, such as platinum, iridium, and palladium, can be used, and practical examples thereof are described in U.S. Pat. Nos. 2,399,083 and 2,448,060, and British Pat. No. 618,061.
It has been strongly desired to increase the sensitivity of silver halide emulsions as high as possible, and for that purpose, attempts to increase the sensitivity of silver halide emulsions by using further new sensitizing techniques have been made.
For example, U.S. Pat. Nos. 3,625,697; 3,622,329; and 3,574,709 disclose a technique of using a well-known sensitization technique together with certain organic thioether compounds.
However, these attempts which have been made have not been completely satisfactory, and are accompanied by defects, for example, the chemical ripening can not progress to obtain sufficient sensitivity since fog increases during the progress of the chemical ripening, and the extent of the progress of chemical ripening is changed, which makes it difficult to control the extent of chemical ripening. Also, the sensitivity itself thus obtained is not always satisfactory.